Marek's disease (MD) is an oncogenic lymphoproliferative disorder of chickens which results in T-cell lymphomas and peripheral nerve demyelination and is a major cause of economic loss to the poultry industry.
Marek's disease virus (MDV) has been identified as the etiologic agent of MD.
A prototype MD vaccine consists of Herpesvirus of Turkeys (HVT), a serotype 3 MD virus originally isolated from turkeys. Its lack of pathogenicity, oncogenicity, its good replication in vivo and in vitro, availability as cell-free and cell-associated preparations and high protective efficacy have established HVT as a very successful and widely used safe vaccine for the effective control of Marek's disease in poultry.
At present, in general, animals can be protected against infection of pathogenic microorganisms with live or inactivated vaccines or by vaccines derived from subunits of the relevant pathogens.
However, these types of vaccines may suffer from a number of drawbacks. Using attenuated live viral vaccines always involves the risk of inoculating animals with inadequately attenuated pathogenic microorganisms. In addition the attenuated viruses may revert to a virulent state resulting in disease of the inoculated animals and the possible spread of the pathogen to other animals.
Inactivated vaccines generally induce only a low level of immunity, requiring additional immunizations. Furthermore, the neutralization inducing antigenic determinants of the viruses may become altered by the inactivation treatment, decreasing the protective potency of the vaccine.
Moreover, a problem with combined live viral vaccines is the mutual influence of the antigenic components resulting in a decrease of the potency of one or more of the constituting components.
A recombinant or naturally derived subunit vaccine also displays a number of disadvantages. First, a polypeptide subunit presented to the immune system as a nonreplicating structure often does not elicit long-lasting immunity requiring also the presence of an adjuvant. Secondly, a presentation as a replicating structure can elicit immunity more efficiently than can a presentation as a subunit structure.